From DE 36 36 936 A1 a pressure diecasting or injection molding machine has become known, in which a plurality of sensor-controlled evaluation circuits are provided which serve to control or regulate the individual parameters of the pressure diecasting or injection molding machine. For this purpose this machine has a multiplicity of sensors, which are situated in the drive unit for the casting or molding cylinder, in the casting or molding cylinder itself, in the gate part and in the cavity of the die or mold. The measurement signals of these sensors serve to control or regulate, for example, the prefilling phase, the die filling phase and the dwell phase of a metalworking pressure diecasting machine.
In the processing of metal or plastic melts in dies or molds it consequently becomes frequently necessary to detect the position of the front of the molten material in the injection mold or die at a particular moment in the course of the casting process. If the exact position of the melt front is known, various parameters of the casting process can be controlled or regulated in dependence on this position, for example with the aid of a computer system, and this leads, for example through uniformity of metering accuracy, to a higher quality of the parts to be produced and thus to improved economy of the casting or molding process. In addition, the position of the melt front, in conjunction with the position of the casting or injection piston, can give information for metering control.
Particularly for the detection of the melt front within the gate channel or the die or mold cavity, sensors have become known from the previously mentioned publication, which act as a contact bridge in conjunction with the electrically conductive metallic casting material. Such contact pins have the disadvantage that only reliably electrically conductive materials are recognized, while in addition drops running ahead or bridging films lead to faulty measurements.
Experiments with thermocouples instead of contact pins have shown that this arrangement does not satisfy the requirements imposed either in respect of senor life or in respect of accuracy of measurement.
The object on which the invention is based is that of avoiding the disadvantages of the prior art and in particular proposing an apparatus for detecting the melt front in pressure diecasting or injection molding machines which is simple and inexpensive in construction and which supplies reliable measurement results.